What should I do regarding a person who deceived me and associated with me under the pretext of marriage, then exposed me after his affair was uncovered?

If the legitimate marriage contract (nikah (definition)) was concluded, fulfilling all conditions, then he is your husband. What you both did is not a sin, and he does not have the right to annul the contract except through divorce. You are entitled to your full Mahr (dowry), and he is not permitted to tarnish your reputation, as this involves the prohibition of slandering Muslims' honor. If the legitimate contract was not concluded, then what you both did is a grave sin and a great indecency. You both must offer sincere repentance, conceal each other's faults, and seek guidance from those in authority to restrain him from causing harm.